1. Field of the Invention
The present invention relates to vapor recovery systems used in connection with fuel dispensing apparatus, and, more particularly, to a method and system for monitoring the recovered vapor emissions and adjusting the flow rate of pumped vapors to eliminate excess collection of air.
2. Description of the Related Art
The evaporative properties of liquid fuel creates a vapor condition within vehicle fuel tanks in which a volume of volatilized fuel overlies the volume of liquid fuel. During the course of refueling the vehicle, the gasoline flowing into the fuel tank will displace the existing fuel vapor and cause environmentally hazardous vapors to be forced out of the tank and into the atmosphere unless precautionary measures are followed to collect and dispose of the discharged vapors. Rising public awareness of the adverse environmental and health consequences of vapor pollutants has prompted governmental authorities to require that fuel dispensing systems be designed to eliminate the release of vapors into the atmosphere by collecting the vapors for storage and possible recycling. These concerns have led to the development of various systems designed to collect and return the fugitive vapor emissions to a storage tank, which typically corresponds to the on-site underground facility located at the service station where the fuel supply is maintained. The recovered vapors may be further transported to a processing site where the vapors are returned to liquid form in a recycling operation or otherwise disposed of by appropriate means.
One class of conventional vapor recovery systems utilizes a vacuum pump to assist in the collection of fuel vapors and their subsequent transfer to a storage tank. The vacuum pump draws fugitive vapors into an intake line that conveys the collected vapors back to the storage tank. The aspirating action generated by the vacuum pump is normally sufficient to capture the vapor emissions, thereby obviating the need for any sealing element such as a bellows member that is otherwise used to surround the nozzle and seal the vapor recovery passageway to the filler neck of the tank. The inlet port of the vapor intake line need only be disposed in close proximity to the filler neck of the fuel tank from where the vapors emanate.
It is critically important in all such vacuum-assist vapor recovery systems that the volume of gaseous mixtures drawn in through the vapor recovery vacuum inlet closely approximate the volume of vapor being displaced by the gasoline flowing into the fuel tank. If the volume of vapor being collected is less than that being displaced, the non-recovered portion will dissipate into the atmosphere. Conversely, if the volume of vapor being collected is greater than the volume being discharged from the tank, the excess volume will consist of atmospheric air that is recovered along with the vapors. Both conditions are to be avoided. Several configuration have been proposed that focus upon making calculated adjustments to the flow rate generated by the vapor pump based upon measurements produced by sensing apparatus that monitor the fueling and vapor recovery operations.
U.S. Pat. No. 5,355,915 to Payne discloses a vapor recovery fuel dispenser including a vapor pump driven by an electric motor. Sensors are provided to generate pulse train signals representative of the flow rate of the liquid fuel pump and the vapor pump. A controller is provided to control the speed of the vapor pump based upon a comparison of the flow rates of the liquid fuel pump and vapor pump, as indicated by their respective pulse train signals. The controller also monitors whether the liquid pump is operating, whether the vapor pump motor is operating, and the electrical current to the vapor pump motor. Appropriate action is taken by the controller to disable the vapor pump when the parameters being monitored indicate a disabling or error condition.
U.S. Pat. No. 5,417,256 to Hartsell et al. discloses a fuel dispensing system including a vapor pump that provides a vacuum suction along a main vapor recovery path. The system further includes a branch conduit coupled to the main vapor path to provide a branch vapor recovery path, and an adjustable vapor flow valve integrated into the branch conduit and having an adjustable opening, that varies the impedance of the vapor recovery path. A fuel sensor is provided to generate a signal representative of the flow rate of the fuel being dispensed, while a vapor flow sensor supplies a signal indicative of the actual vapor flow rate. A controller is responsive to the flow rate signal for the dispensed fuel and generates a control signal to adjust the vapor flow valve so that the actual vapor flow rate is equalized to a required or desired vapor flow rate calculated on the basis of the liquid fuel flow rate and a ratio-based comparison between the temperatures of the liquid fuel and the atmosphere.
U.S. Pat. No. 5,040,577 to Pope discloses a fuel delivery system comprising a vapor recovery assembly including a recovery pump that draws fugitive vapor emissions through a recovery tube in accordance with a controllable volumetric flow rate. A microprocessor is provided to control the recovery pump so that is withdraws vapor at a flow rate equal to the volumetric flow rate of the fuel delivery pump that regulates the dispensing of fuel. Further adjustments to the vapor flow rate may be made in response to data provided by pressure sensors indicating the hydraulic pressure at the inlet side of the pump.
U.S. Pat. No. 5,269,353 to Nanaji et al. discloses an apparatus for pumping recovered vapor in a vapor recovery liquid fuel dispenser having a vapor passage used to retrieve fuel vapors. The apparatus includes a vapor pump operative to induce vapors to enter and move along the vapor passage and through a vapor pump inlet to a vapor pump outlet. The vapor pump is characterized by a flow rate correlated to a specified operating speed that is inversely proportional to the pressure differential existing between the vapor pump inlet and outlet. Sensors are provided to generate signals representative of these vapor pump pressures. A transducer generates a liquid fuel flow signal indicative of the flow rate for the fuel being dispensed. Electronic circuitry is provided to derive the vapor pump flow rate from the pressure differential and then implement the appropriate adjustments to the operating speed of the vapor pump so that the vapor pump flow rate is equalized with the liquid fuel flow rate.
The above systems are almost exclusively concerned with adjusting the vapor flow rate on the basis of measurements that are neither directly probative nor specifically indicative of the hydrocarbon concentration of the recovered vapors. Any needed adjustments are instead made in response to direct measurements of the volumetric flow rates of the liquid fuel being dispensed and the withdrawn vapors, which measurements are then used to determine the specific change that is required in the vapor pump operating speed in order to match the vapor flow rate to the liquid fuel rate. The overall purpose of tracking the vapor flow rate to the liquid fuel rate is to ensure that the volumetric quantity of retrieved vapor is the same as the volumetric quantity of vapor being displaced by the dispensed fuel. However, the only true measure of performance is based upon whether and to what extent excess air is being prevented from being pumped into the vapor recovery line along with the vapor emissions. Measured against this performance standard, the accuracy of the above systems is not verifiable, and is potentially inexact, since no measurements are obtained of the retrieved vapor to determine its hydrocarbon or air content.
U.S. Pat. No. 5,507,325 to Finlayson discloses a vapor recovery system for fuel dispensers that incorporates a measurement of a vapor-to-air ratio in its control apparatus regulating the vapor retrieval process. Vapors displaced from the tank are collected through a vapor intake and pumped by a variable rate vacuum pump into a vapor storage tank. A flow meter produces a signal representative of the liquid fuel flow rate. An array of vapor-to-air ratio sensors are provided to produce signals representative of the vapor-to-air ratio as measured at a variety of locations that are proximate to the tank opening. The sensors used by the Finlayson reference operate specifically to detect the physical presence of fuel vapors in the sensing environment. A controller is provided to determine a base collection rate (based on the liquid fuel flow rate) at which to operate the vapor pump, which base pump rate is then adjusted according to the signals generated by the vapor-to-air ratio sensors in order to minimize the amount of fuel vapor that escapes into the atmosphere and to minimize the amount of air contained in the gaseous mixture that is drawn along the vapor intake line.
The vapor recovery system of Finlayson is an advance over the systems described above because it provides a means by which the compositional content of the recovered emissions (i.e., vapor versus air) can be directly measured. This permits a more accurate evaluation of whether the vapor pump is inducing the proper volumetric flow of fugitive emissions into the recovery line. However, there are problems attending the Finlayson system which stem from the fact that the sensors are specifically designed to detect the presence of fuel components. Vapor condensation within the intake line is a recurring problem that results when differentials in temperature and pressure within the vapor recovery system reach threshold conditions. The accumulation or even transient deposition of condensed fuel vapors on fuel-detecting sensors will produce false readings of the fuel content in the monitored environment and lead to improper adjustment of the vapor pump rate. Additionally, since only fuel components are being directly detected, any determination of the air content, which alone provides the truest measure of the efficiency of the vapor recovery process, is based on a calculation and not an actual physical reading.
What is therefore needed in the art is a system that monitors the fugitive vapor emissions displaced from a tank during refueling and that adjusts the vapor recovery rate based on direct measurements of the concentration of air in the monitored environment, from which a hydrocarbon concentration can be derived and used to appropriately vary the operating speed of the vapor pump.